Cell Biology- Chapter 11

The Lipid Bilayer

• Separates intracellular spaces from the outside environment

• Regulates exchange within a cell

• Naturally impermeable to most molecules

Cell Membrane Characteristics

• Not a rigid barrier, but a living structure

• Functions:

  • Evaluate and communicate with the environment

  • Acquire needed molecules & discard unnecessary ones

  • Traverse its environment

Internal Membranes & Compartmentalization

• Organelles isolated by their own membranes

• Allows different reactions to occur in separate regions

• Examples:

  • ER: Lipid sorting, membrane-bound/secreted protein synthesis

  • Peroxisome: Oxidation of long fatty acids

  • Lysosome: Hydrolysis of macromolecules

  • Golgi: Protein modification & trafficking

  • Mitochondria: RedOx reactions, energy extraction

Membrane Composition

• Consists of phospholipids and proteins

• Some proteins act as rafts isolating specific regions

• Amphipathic nature helps form bilayers in water

Phospholipid Behavior in Bilayers

Dynamic positioning:

• Lateral diffusion

• Flexion

• Rotation

• Flip-flop (facilitated by flippase)

Membrane Fluidity

• Function of temperature & composition

• More unsaturated lipids = increased fluidity

• Cholesterol buffers fluidity against temperature changes

Membrane Assembly

• Begins in the ER

• Phospholipids synthesized in the cytosol, inserted into ER membrane

• Scramblase randomizes lipid distribution in the ER

Asymmetric Lipid Distribution

• Occurs at the Golgi

• Maintains orientation during transport

• Glycolipids positioned on extracellular leaflet

• Phosphatidylserine flips to extracellular leaflet during apoptosis

Membrane Proteins & Functions

Types:

• Transporters

• Anchors

• Receptors

• Enzymes

Membrane Protein Association

• Integral:

  • Transmembrane

  • Monolayer-associated lipid-linked

• Peripheral:

  • Protein-attached

Membrane-Spanning Protein Domains

• Rich in hydrophobic residues

• Form alpha-helices or beta-sheets

• Hydrophilic regions create aqueous pores

Studying Membrane Proteins

• Detergents can release membrane proteins

• Methods:

  • X-ray crystallography

  • Cryo-electron microscopy

  • NMR spectroscopy

• Example: Bacteriorhodopsin (light-driven proton pump)

The Cell Cortex

• Network of proteins supporting cell shape

• Cytoskeleton attachment to membrane proteins

• Actin complexes linked by spectrin tetramers

Membrane Protein Movement

• Measured by FRAP (Fluorescence Recovery After Photobleaching)

• Types:

  • Unrestricted

  • Confined

  • Tethered

Protein Movement Restriction Mechanisms

• Tethering to intracellular/extracellular proteins

• Interactions with adjacent cells

• Diffusion barriers at tight junctions

Epithelial Cell Polarity

• Tight junctions create diffusion barriers

• Separate apical & basolateral domains

Glycosylation & Cell Recognition

• Plasma membrane coated with sugars

• Glycosylation occurs on:

  • Lipids

  • Transmembrane proteins

  • Peripheral proteins

• Proteoglycans contain glycosaminoglycans (GAGs)

Role of Glycocalyx

• Protects cells from damage

• Facilitates cell recognition

• Inflammatory responses involve lectins binding to glycosylated cells

Experimental Techniques

• Purified proteins inserted into artificial bilayers to study function

• Liposomes useful for studying transport proteins